The present invention relates to a process for removing sulfur oxides from gases by adsorption. More particularly, the present invention is characterized in that sulfur oxides contained in various combustion waste gases and waste gases from a Claus plant are removed by adsorption with an adsorbent comprising a titanium oxide and iron oxide and/or copper oxide as principal component(s).
Various sulfur oxides are contained in combustion waste gases from boilers in a power plant, a refining plant, a steel making plant and a chemical plant or in a waste gas from sulfuric acid industry or in a tail gas from Claus furnace wherein sulfur is recovered. Sulfur oxides comprises mainly SO.sub.2 and SO.sub.3. The sulfur oxides will be referred to as SO.sub.x hereinafter in the specification.
SO.sub.x is a principal cause for air pollution. A legislation restricting the amount of the pollutant released to the atmosphere is expected to be tightened for environmental protection. Several techniques for removing SO.sub.x from a waste gas have been put to practical use. One of the techniques comprises a process wherein SO.sub.x is absorbed with a calcium carbonate slurry and then it is oxidized to form calcium sulfate (gypsum). Another technique comprises a process wherein SO.sub.x is absorbed with an alkaline absorbing liquid, the same is reacted with calcium oxide to form calcium sulfite and calcium sulfite is oxidized to obtain gypsum. According to those processes, gypsum is formed in a large amount. Further, development of a process for treating the absorbing liquid (for example, by-produced Glauber's salt) is required.
As a dry SO.sub.x removal technique, there has been a process wherein SO.sub.x in a waste gas is adsorbed by a solid adsorbent. SO.sub.x thus adsorbed is desorbed from the adsorbent with a reducing gas to obtain a gas containing a relatively high concentration of SO.sub.x and elemental sulfur is recovered from the desorbed gas. This process has merits that sulfur thus obtained is of utility value and that this process is free from a problem of treating waste liquid.
As for a dry process, for removing SO.sub.x from a waste gas there has been known a process wherein SO.sub.x is absorbed or adsorbed by using a solid adsorbing material comprising iron oxide supported on an alumina carrier [David T. Clay and Scott Lynn, Reduction and removal of SO.sub.2 and NO.sub.x from Simulated Flue Gas using Iron Oxide as Catalyst Absorbent, AIChE Journal 21 (1975) 466]. There has also been known a process wherein a copper oxide adsorbent supported on an alumina carrier is used (Japanese Patent Publication No. 17951/1974).
However, according to broad supplementary examinations, the inventors have found that adsorbing capacities of the iron oxide and copper oxide adsorbents supported on alumina are reduced gradually while they are used for a long period of time. Particularly, if SO.sub.3 (sulfuric acid mist) is contained in a gas to be treated, a part of the alumina carrier is converted to aluminum sulfate, resulting in loss of its function rapidly.
A boiler waste gas contains, in addition to SO.sub.x, nitrogen oxides (NO and NO.sub.2 ; hereinafter referred to as NO.sub.x) and oxygen gas. It has been found that, when NO.sub.x is reduced to nitrogen with ammonia as the reducing agent and SO.sub.x is adsorbed by the CuO or Fe.sub.2 O.sub.3 adsorbent-catalyst simultaneous by, the adsorbent supported on alumina carrier is easily deteriorated.
Further, since the CuO or Fe.sub.2 O.sub.3 adsorbent supported on alumina carrier has not so high adsorption velocity as expected, a large amount of the absorbent is necessitated, and consequently a large amount of energy is wasted for transportation of the gas.